Non-invasive measurement of blood pressure (BP) using cuff-based methods provides adequate data for many applications in medicine. However, cuff-based methods have some disadvantages that limit their utility in certain clinical situations. For example, as cuff-based methods are by definition a discontinuous measurement of blood pressure, in BP that occur between measurements are missed. Additionally, the inflation of the cuff may disturb the patient and the consequences of these disturbances are alterations of the BP and undesired arousal of the patient during sleep.
An alternative approach relates to a continuous, non-invasive and indirect measurement of BP based on the vascular transit of a pulse pressure wave. A number of variables been described in the art as relating, at least in general terms, to arterial blood pressure. In one example, pulse transit time (PTT) is defined as the time for a pressure wave launched by contraction of the left ventricle of the heart into a patient's arterial system to travel between two arterial sites. PTT may be considered to be a function of arterial compliance (or “stiffness”), the propagation distance of the pressure pulse being measured, and arterial blood pressure, and has been shown in a number of studies to correlate to systolic (SYS), diastolic (DIA), and mean (MAP) blood pressure. Similarly, pulse wave velocity (PWV) is a measure of the velocity at which the same pressure wave moves between two arterial sites. PWV is often described as a direct measure of arterial compliance, but is also a function of vessel dimension and arterial blood pressure.
Travel of a pulse pressure wave through a portion of the vasculature may be measured with a vital signs monitor that includes separate modules to determine both an electrocardiogram (ECG) and pulse oximetry (SpO2). During a measurement, multiple electrodes typically attach to a patient's chest to determine a time-dependent ECG component characterized by a sharp spike called the ‘QRS complex’. The QRS complex indicates an initial depolarization of ventricles within the heart and, informally, marks the beginning of the heartbeat and a pressure pulse that follows. SpO2 is typically measured using a sensor that attaches to a patient's finger, and includes optical systems operating in both the red and infrared spectral regions. A photodetector measures radiation emitted from the optical systems that transmits through the patient's finger (although other body sites, e.g., the ear, forehead, and nose, can also be used in place of the finger).
During a measurement, a microprocessor analyses both red and infrared radiation detected by the photodetector to determine the patient's blood oxygen saturation level and a time-dependent waveform called a photoplethysmograph (‘PPG’). Time-dependent features of the PPG indicate both pulse rate and a volumetric absorbance change in an underlying artery caused by the propagating pressure pulse. A number of publications describe the relationship between PTT and blood pressure. For example, U.S. Pat. Nos. 5,316,008; 5,857,975; 5,865,755; and 5,649,543 each describe an apparatus that includes conventional sensors that measure an ECG and PPG, which are then processed to determine PTT. Gesche et al., Eur. J. Appl. Physiol. 112: 309-15, 2012, discloses a similar apparatus for relating PWV to systolic blood pressure. Each of these publications is hereby incorporated by reference in its entirety.
To account for patient-dependent properties, such as arterial compliance, pulse pressure wave-based measurements of blood pressure are typically ‘calibrated’ using a conventional blood pressure cuff. Typically during the calibration process the blood pressure cuff is applied to the patient, used to make one or more blood pressure measurements, and then left on the patient. Going forward, the calibration blood pressure measurements are used, along with a change in PTT or PWV to determine the patient's blood pressure and blood pressure variability.